Selective amplifications of LP01 fundamental mode and higher order modes LP11 and LP01 are demonstrated in a double-pass Nd-doped LMA fiber amplifier operating at 910 nm. A multimode core fiber Bragg grating is employed to select a single guided mode by simply adjusting the wavelength of the seed signal. Although the M2 parameter of the output beam from the amplifier was ~2.5 in a single-pass configuration, a double-pass configuration with LP01 mode selection reduces the value of the M2 parameter to 1.06 in spite of the multimode nature of the core (V~5). In addition, it is shown that this amplifier configuration permits to lower both the power saturation and the parasitic emission at 1060 nm, which consequently increase the pump-to-signal conversion efficiency at 910 nm.
© 2017 Optical Society of America
Large mode area (LMA) fiber design is commonly used to reduce nonlinear effects in high-power or single-frequency laser regimes. Indeed, non-linear effects, and more specifically Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SBS) and Self-Phase Modulation (SPM), are responsible for strong deteriorations of the temporal and spectral characteristics of laser emission propagating in optical fibers. For high peak-power pulsed operation, large core diameters are also necessary to increase energy storage and avoid facet damages. However, while near-diffraction-limited beam quality is most often required for many applications, increasing the core diameter may also imply the propagation of higher order modes (HOMs). Consequently, the amplification of these HOMs leads inevitably to the degradation of the spatial beam quality and can even trigger modal instabilities at high power levels . Several approaches have been explored in order to limit the occurrence of higher order modes in an amplifier or a laser based on a large-core fiber. One of the main approaches consists in filtering out HOMs by applying local or distributed losses selectively on these HOMs. This can be achieved in low-NA step-index fibers by simply bending , tapering the fiber  or by the excitation of the fundamental mode using a mode-matched input seed beam . However, with increasing core sizes, the effective-index spacing between modes is reduced and the HOM discrimination by these techniques may not be sufficient, especially in high-power regimes. Other more advanced solutions are based on gain modal filtering , multi-trench in the fiber index profile , Bragg fibers  or microstructured fiber designs such as photonic crystal fiber (PCF) , photonic bandgap fibers (PBGF)  or leakage channel fibers (LCF) . As for example, large pitch fibers (LPFs), a modified version of PCF, allow the delocalization of HOM out of the core area, which ensures single-mode operation in high-power laser regimes even with mode field diameters exceeding 100 μm .
However, although most of these techniques have proved to be efficient for high-power Yb-doped fiber laser systems, the fabrication processes remain complex and are not often applied to other rare-earth (RE)-doped fibers. Hence, other techniques of HOM suppression are still of great interest, especially for novel RE-doped fiber development that requires multi-step iterative process in order to optimize the geometry or the core-composition. For that purpose, a convenient solution is the mode-matched injection technique, which can be applied to any multimode core fiber provided that only LP01 mode is excited. However, free-space injection or the use of mode-field adaptor does not provide sufficient mode selectivity to suppress HOMs amplification in high power regime.
Recently, a novel method of modal selection was applied to a multimode Raman fiber laser at 954 nm by using a cavity mirror made of a FBG inscribed in the central part of the fiber core, thus enabling a nearly diffraction-limited output beam (M2 ≤ 1.27) . Another flexible but highly selective method consists in using a conventional multimode fiber Bragg grating (FBG) with a geometry similar to the RE-doped fiber. A multimode FBG is intrinsically characterized by a reflection spectrum which is dependent on the propagation mode. This technique was already applied to a Thulium doped multimode fiber laser and permits to demonstrate single mode operation while the V parameter of the fiber core was ∼6.3 at a wavelength of 2 μm . In order to ensure efficient mode filtering, a second narrow band mirror such as a VBG is necessary to force the laser oscillating at the Bragg wavelength of the LP01 mode. To the best of our knowledge, this mode-filtering method was so far only applied to fiber laser configurations.
In this paper, we demonstrate that efficient FBG mode selection can be achieved in a Master-Oscillator Power-Amplifier (MOPA) based on a double-pass scheme. For that purpose, a tunable laser diode emitting near 910 nm was amplified in a double-clad Nd-doped fiber (NDF) with a multimode core of 20 µm. A specific mode (LP01 or HOMs) can be selectively amplified by matching the wavelength of the laser diode to the respective mode Bragg wavelength. In addition, it is shown that this scheme is particularly well-adapted to the amplification near 900 nm in a neodymium-doped double-clad fiber for which a large core is required to avoid parasitic lasing at 1060nm . Experimental results indicates that the gain saturation and the reduction of the amplified stimulated emission (ASE) at 1060 nm are much more effective in a double-pass scheme compared to a single-pass amplification.
2. Spectral and modal properties of the multimode FBG
The number of modes supported by a large core optical fiber at a given signal wavelength only depends on its refractive index profile. Each of these guided modes is characterized by an effective refractive index neff which can be calculated from the index profile. Generally, HOMs have a greater spatial extension, which implies a lower value for neff compared to the fundamental mode. Consequently, a Bragg grating written into a multimode fiber core supporting N modes is characterized by N Bragg wavelength λFBG, each depending on the grating period Λ and on the neff value of the guided mode. The Bragg wavelength can be thus written as λFBG = 2 neff Λ. Assuming that the spectral width of the FBG is smaller than the wavelength separation between two consecutive modes, only one mode can be reflected by the FBG whereas the other modes are transmitted. Therefore, mode selection can be achieved by simply adjusting the wavelength of a spectrally narrow incident signal.
The FBG used in these experiments was a conventional uniform Bragg grating inscribed by UV laser in the fiber core. The fiber has a double-clad structure with a core diameter of 18 μm and a clad diameter of 80 μm. The linewidth of the FBG was estimated to be ~0.2 nm (FWHM) for the fundamental mode while its reflectivity was 90% at 910 nm. The spectral properties of the FBG were first characterized in order to verify its ability to reflect a specific guided mode. The transmission spectrum of the FBG was measured by using a broadband Amplified Stimulated Emission (ASE) source near 910 nm based on a Nd-doped fiber pumped at 808 nm. The measured spectrum is presented in Fig. 1.
From this spectrum, it is possible to identify three well-separated peaks, corresponding to the fundamental mode LP01 and the first two HOMs LP11 and LP21. A fourth peak, which partially overlaps with the third peak, also appears and might correspond to the LP02 mode. Due to the residual light propagating in the fiber clad of the FBG, the reflectivity of each peak could not be precisely determined. The identification of the four modes with respect to measured Bragg wavelengths was confirmed by the calculations of the effective indexes from the measured refractive index profile of the fiber using a mode solver. The results are summarized in Table 1 and show the good agreement between the calculated and experimental Bragg wavelengths.
3. Experimental set-up and results
The schematic of the MOPA configuration is shown in Fig. 2. An external-cavity diode laser (ECDL) was used as a narrow-linewidth seed source and could be tuned over a few nm near 910 nm by rotating a diffraction grating in Littrow configuration. The power-amplifier was based on a Nd-doped double-clad fiber with a large core and a small inner cladding to increase the gain at 910 nm while limiting the onset of ASE at 1060 nm . The signal injection end of the fiber was angle-cleaved and the other end was spliced to the FBG. The Nd-doped fiber and the FBG have both an inner clad diameter of 80 μm (NA = 0.45) and a 18 μm core with similar refractive index profiles to avoid differences in modal content between the two fibers. To further reduce the parasitic emission at 1060 nm, the length of the Nd-doped fiber was as short as 4 m, corresponding to an unsaturated absorption of 70% at 808 nm. The amplifier was pumped through the angle-cleaved end of the FBG with a maximum launched power of 13 W at 808 nm. The seed signal delivered by the ECDL and the amplified signal after a double-pass in the Nd-doped fiber were separated by polarization by means of a Glan-Taylor beamsplitter cube as well as λ/4 and λ/2 waveplates. The necessary 90° polarization rotation for the backward signal was achieved by the λ/4 waveplate whereas the λ/2 waveplate was used to compensate polarization rotation due to stress-induced birefringence in the fibers. Moreover, the amplifier could also be operated in single-pass configuration by tuning the ECDL to a wavelength which is transmitted though the FBG (~911 nm during experiments). A low-pass filter at 1000 nm was inserted before the NDF in order to monitor the backward 1060 nm emission (reflected by the filter).
It can be noted that an all-fiberized set-up could be achieved by using a fiber optic circulator in place of the bulk optical components of the free-space part of this arrangement. However, it is not certain that such a multimode fiber component would preserve the modal content. Hence, more reliable but lossy set-up was preferred to validate the proof-of-concept of the technique.
The modal selection by wavelength tuning of the laser diode was first verified by removing the NDF from the experimental set-up shown in Fig. 1. Hence the beam from the laser diode was directly injected in the FBG and the far-field intensity profile of the backward reflected beam was recorded by using a CCD camera. By tuning the ECDL at each of the four previously measured Bragg wavelengths, we have successfully checked that guided modes can be individually selected with no (or limited) mode mixing, as shown in Table 2.
n the specific case of LP02, stable mode selection was also more difficult to achieve due to the small Bragg wavelength difference with the more robust LP21 mode. As can be expected, the FBG only acts as a selective mode filter and, therefore, the intensity of the reflected mode strongly depends on the modal content imposed by injection conditions at the input fiber end. Hence, the output amplified power may vary with the launching conditions of the signal. Alternatively, a fiber mode scrambler could be used for better mode equilibrium. By selecting LP01 mode, M2 parameter of the reflected beam was equal to 1.05, which confirms the efficiency of the modal filtering by the FBG.
The amplification of a single LP mode was then investigated after the NDF was fusion spliced on the FBG without any special optimization in terms of mode coupling. By selecting LP01 mode, an amplified output power of 2.2 W was measured at 910 nm after polarization separation for an input seed power of 50 mW and a pump power of 13 W. The M2 parameter, equal to 1.06, confirms once again the selective amplification of the LP01 mode without mode mixing. As a main drawback, a power of 650 mW is transmitted through the Bragg grating and corresponds both to the HOMS amplified after a single pass and residual transmission of the FBG. In addition, it was verified that amplified ASE near 900 nm from HOMs is not significant as the amplifier is operated in the strongly saturated regime. Indeed, the output spectrum of the amplified HOMs only shows the narrow peak at the laser diode wavelength. This partly explains the average conversion efficiency (17%) of the amplifier, while slope efficiencies of 50% were already observed in laser configurations with similar NDF . However, this difference is mainly due to the low effective pump absorption of the short NDF length and other component losses, while HOMs amplification only decreases the efficiency by a few percent. The gain in terms of LP01 power in the second pass was estimated to ~12 dB, which also means that most of the output power is extracted from the amplifier during this second pass. The evolutions of both amplified output power near 910 nm and ASE power at 1060 nm versus the launched pump power are given in Fig. 3 in the cases of double-pass (LP01 mode operation, Fig. 3(a)) and single-pass multimode amplification, Fig. 3(b). When operated in single-pass configuration (signal wavelength near 911 nm), the M2 factor of the amplified beam was comprised between 2 and 3 depending on the injection conditions in the fiber, which is consistent with the multimode nature of the core. In addition, maximum output power was limited to 1.5 W due to the rapid increase of ASE power at 1060 nm. Indeed, in this configuration, the low and inhomogeneous saturation of the gain along the fiber greatly favors the emission at 1060 nm for high pumping powers. On the contrary, the ratio between the output powers at 910 nm and 1060 nm does not increase with the pump power in double-pass configuration, which tends to show a higher level of gain saturation all along the fiber [Fig. 3(a)]. The comparison of the gain evolution curves of the Nd-doped fiber amplifier in both configurations also confirmed that much lower seed power was required for efficient power extraction in double-pass configuration as only 2 mW was necessary to reach gain saturation (against 30 mW) at the maximum pump power.
Finally, by selecting LP11 and LP21 modes, amplified output powers of only 1.5 W and 1.1 W were respectively measured for a pump power of 11 W. The beam intensity profiles were measured for these two modes and appeared to be almost identical to the LP11 and LP21 profiles presented in Table 2. The lower output powers could be related to increasing propagation losses and/or lower gain for higher order modes in the LMA amplifier. However, further investigations would be needed to fully understand the origin of lower gain for HOMs.
We have demonstrated the principle of mode selection by a fiber Bragg grating in a double-pass fiber amplifier. This method is used to generate a diffraction-limited beam in a MOPA system based on a multimode fiber core. In the specific case of a Nd-doped LMA fiber, we have shown that the double-pass amplification strongly reduces the saturation power at 910 nm while conversion efficiency is increased compared to single-pass amplification. In addition, the first two HOMs can be selected and amplified individually by simply adjusting the wavelength of the seed source. This technique could potentially be applied to any CW or nanosecond pulsed MOPA based on rare-earth doped large-core fibers. It is also expected that the laser conversion efficiency could be improved in the near future with polarization-maintaining fibers and/or all-fiber components.
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